This invention relates generally to variable frequency electric motor systems and more particularly to those utilizing variable frequency induction motors.
There exist in many industrial and commercial applications a requirement for variable frequency or variable speed drive electric power systems. Typical applications are found in pumping systems, fans, textile agitators and textile dying devices. Initially, the increased use of electrical power to fulfill such needs resulted in application of the DC motor as the primary source of such variable frequency power. This was due primarily to the ease with which the speed and load characteristics of a DC motor are controlled. However, it has become apparent to practitioners in the art that several significant advantages favor AC motors rather than DC motors in such power systems. The smaller size and weight of AC motors for a given power requirement is particularly significant. Also, AC motors exhibit greater torque-to-inertia ratios (i.e. better speed of response) and better environmental compatibility. The latter results in a reduced need for enclosure in high humidity or chemical vapor environments. Frequently, DC motors with their inherently vulnerable brush contacts and commutators are incompatible with such environments and must therefore be enclosed. Finally, the excellent low speed characteristics of variable frequency AC motor systems often eliminate the need for costly reduction gear systems used in low speed DC motor drive systems.
These considerations as well as others have led practitioners in the art to develop variable frequency drive induction motor systems and while various different systems have been developed, most include the same basic elements. First, an AC-to-DC converter is used to change the available fixed-frequency AC main power to a DC voltage. Next, an inverter is used to reconvert the derived DC power to a variable frequency AC voltage. And finally, a frequency control is operatively coupled to the converter to change the frequency of AC power produced thereby which in turn varies the motor frequency.
The majority of motor controls developed tend to be either one of two types. The first, known by the names "square wave inverter", "constant flux" or "constant volts per Hz" system utilizes a control in which the output voltage of the DC converter and the frequency of the DC-to-AC inverter are varied in tandem. The second system is known as the "pulse width modulated system". The AC to DC conversion bridge in pulse width systems produces a fixed DC output voltage rather than the variable DC characteristic of constant flux systems. The inverter typically includes a "chopping circuit" generally comprising a plurality of SCR type devices which operate to chop or serrate the fixed DC voltage. Motor frequency control is achieved by varying the duty cycle of the chopping circuit and thereby the effective voltage applied to the DC-to-AC inverter system.
While these systems have thus far provided considerable improvement over the variable speed DC power systems, several problems in their operation have arisen. One source of several problems is the production of harmonics, or higher frequency, power components during the conversions of power from AC to DC and DC back to AC. This harmonic content in the generated power waveform causes increased I.sup.2 R or heat losses as well as increased magnetic losses within the core and windings of the motor. These losses become of even greater concern in operations wherein low rotor speeds are utilized due to the reduced cooling wind produced at low rotor speeds. In critical operations, auxiliary cooling systems are utilized to carry off heat during low speed operation. These and other problems have produced an increased awareness of the need within the art to improve motor efficiency and thereby reduce the heat produced during motor operation.
One aspect of these heat problems arises due to the tendency of present variable speed systems to apply a maximum or near maximum power level to the motor under all load conditions. Variations of torque or load requirements are met by changes of motor power factor and slip angle in accordance with motor characteristics. This method of operation causes "over flexing", excessive heat production, and increased mechanical stresses on motor laminations. Practitioners in the art have recognized these limitations and have been led thereby to attempt development of systems which include motor power factor in the operational response of the system. However, thus far the need remains for a more efficient variable speed induction motor drive control system.